Wow... that is really massive, you will be confused where to begin with. I got FX850 and had IE800, after two weeks even now not able to understand what I like more from 850 when compared to 800.
IE800 is ultra fast when compared to FX850, the way it recovers after an hard hitting bass, for next one, is amazing. Imaging also better IE800, it is also more laid back compared to FX850.
FX850 gives good fight to IE800, it has bigger sound stage and bass hit even more harder but when it has to deliver on fast notes, it does not perform as well as IE800.
However I made only front mod suggested by James, back mod is still pending. Without mod, FX850 is boomy and imaging is pathetic, it is like every musicians is sitting two feet away from me, with no sense of layering IMO.
Thanks to James, his suggest mod make FX850 much better.